1. Field of the Invention
This invention relates to dual-mode filters and particularly to dual-mode filters having dielectric resonators containing apertures.
2. Description of the Prior Art
Dual-mode dielectric resonator filters have been widely used in cellular radios and satellite multiplexers. Although, the use of dielectric resonator technology offers a significant reduction in weight and size in comparison with the waveguide resonator technology, it is known that the spurious performance of dual-mode dielectric resonator filters is not satisfactory for many satellite applications. In satellite multiplexers, improving the spurious performance of such filters will readily translate to higher communication capacity, or cost saving, or further reduction in weight and size or a combination of these factors.
Implementation of dual-mode dielectric resonator filters has been conventionally accomplished by using the resonator configuration shown in FIG. 1, where a solid cylindrical dielectric resonator R, housed within a metallic enclosure M, operates in either the dual HEH.sub.11 mode or the dual HEE.sub.11 mode. It is also known that the proximity of the resonant frequency of the HEE.sub.11 mode to that of the HEH.sub.11 mode interferes with the filter performance causing undesirable spurious response.
The resonant characteristics of the conventional resonator shown in FIG. 1 have been described by K. A. Zaki and C. Chen (IEEE, MTT-34, No. 7, pp. 815-824). A typical mode chart for this resonator is illustrated in FIG. 2 in which the abscissa and ordinate represent the diameter to height ratio and the resonant frequency of the first four modes. Although the location of the spurious response can be controlled by adjusting the resonator dimensions, even with the choice of the optimum dimensions the attainable spurious separation is not adequate to meet the stringent requirements of recent satellite systems. A need has therefore arisen for a dual-mode dielectric resonator with improved spurious performance.
U.S. Pat. No. 4,028,652 issued June, 1977 to K. Wakino, et al. describes a single mode filter having a dielectric resonator containing one or more apertures. Undesirable spurious responses are said to be reduced. The patent does not however suggest the use of dual-mode operation of any of the described resonant structures.
U.S. Pat. No. 4,706,052 issued November, 1987 to Jun Hiattori, et al. describes a single-mode filter design in which a variety of differently shaped, layered and dimensioned dielectric resonators are disclosed and described. While the resonators do not contain apertures, the stated purpose of the invention is to improve the spurious performance of single-mode dielectric resonators operating in the TEH.sub.01 mode. There is no suggestion to use dual-mode operation.